1. Field of the Invention
The present invention relates generally to apparatus for carrying pressure gauges used in measuring oil well formation, or bottom hole pressures. More particularly, the invention relates to apparatus for measuring formation pressure and which is adaptable to be tubing conveyed. Even more particularly, the invention relates to formation pressure measurement apparatus which is tubing conveyed but which offers substantially no pressure drop to the fluids flowing therethrough.
2. Description of the Background
Pressure is probably the most important directly measurable parameter of an oil reservoir. From pressure measurements many other important variables can be derived which can be used in evaluating a well or reservoir, such as method of production, effectiveness of secondary recovery processes, etc. Reservoir Engineers can determine effective permeability, porosity, effective drainage radius, extent of well bore damage (if such exists), continuity of aquifer, oil zone or gas cap from various pressure measurements. These provide transient pressure measurements under draw down, or build-up conditions and interference effects in a shut-in well due to a change in production or injection in another well.
Previous pressure measurements have been made with gauges mounted internally to the drill string; i.e. in the drill string bore. However, these have proved unsatisfactory since they created a substantial pressure drop across the measuring sub or prevented flow altogether. These have likewise proved unsatisfactory since, if formation perforation were desired below such gauges, a drop bar system could not be used since the drop bar could not bypass the gauge sub to reach, for example, the firing mechanism of a perforating gun. Mechanical pressure gauges have been mounted on the outside diameter of a measurement sub; however, these have proved unsatisfactory because of their relatively low sensitivity and accuracy and because of their susceptibility to shock and damage resulting from surge pressure due to the firing of the perforating guns.
Many operators now prefer to have a "full bore" string on all their testing for many reasons. The straight-through bore allows the running of wireline tools in and out of the hole, as well as accommodating the use of the drop bar, tubing-conveyed perforating system. Additionally, pressures may be measured in an essentially uncontaminated state since tests, perforating, etc., may be done in an underbalance condition.
In drill stem testing, special formation test equipment is mounted on the end of the drill string and lowered into the hole to a point above or adjacent to the horizon to be tested.
Drill stem tests are made to ascertain the potential productivity of a penetrated zone, to assess formation damage, to determine native reservoir pressures, and to obtain fluid samples (surface and/or subsurface) without cementing casing or removing drilling fluid from the hole. Such tests can also be used to confirm the effectiveness of water shutoffs and to determine the capability of perforations to admit fluids freely to the hole.
A drill stem test involves the measurement of bottom hole pressures with the formation to be tested alternately closed-in and open to flow to the surface. Therefore, the equipment in the overall assembly consists of the pressure-recording device, the flow-control valves, one or more gland-type packers, and various other safety and control mechanisms. Surface equipment may also include pressure and flow measuring and control devices, along with necessary manifolds, tanks, separators, burners, etc. The test equipment or tool is made up on the drill string and set down on the bottom of the hole. A packer, an expandable hard-rubber sealing element, seals off the hole below it by expanding when weight is set down on it. A valve is opened, and any formation pressure and fluids present enter the tool. A recorder in the tool makes a recording of the formation pressure. Then the packer is released and the tool retrieved back to the surface. By looking at the record of the downhole pressure, a good indication of the characteristics of the reservoir can be obtained.